The structure of a light emitting diode for effectively radiating rays of light, emitted from a light emitting element, outwardly frontward through a metallic reflecting surface is disclosed in numerous patent publications. In those prior arts, the structure can be classified into three types including a first type in that the metallic reflecting layer is vapor deposited on an outside of the casing, a second type in that the metallic reflecting layer is vapor deposited on an inside of the casing, and a third type in that the metallic reflecting layer pre-manufactured is employed.
As a first example in which the metallic reflecting layer is formed on an outside of the casing, light emitting diodes disclosed in the Japanese Laid-open Patent Publications No. 49-82290 and No. 58-82290 are well known in the art. In the light emitting diode of the type shown in FIG. 6, the light emitting element 61 is fixedly mounted on a lead 62a by means of an electroconductive bonding agent 63 and is electrically connected with a lead 62b through a gold wire 64. The light emitting element 61 fitted to the leads 62a and 62b are inserted into a mold having a cavity of a semispherical shape or a parabolic shape and are integrated and encapsulated together by means of a transfer molding technique using a light transmissive resin 65. Thereafter, an outer convex surface of a semispherical shape or a parabolic shape is subjected to a surface coating treatment by means of a metal vapor deposition or plating to form a concave reflecting surface 66 and is subsequently applied an overcoat layer 67 to protect the concave reflecting surface 66. The concave reflecting surface 66 is utilized to reflect rays of light emitted from the light emitting element 61 so that they can be radiated outwardly from a radiating surface 68. With this structure, almost all of the light emitted from the light emitting element 61 can be reflected from the reflecting surface 66 and then radiated outwardly of the light emitting diode through the radiating surface 68.
As a second example in which the metallic reflecting surface is formed on an inside of the casing, light emitting diodes disclosed in the Japanese Laid-open Patent Publications No. 62-269984 and No. 01-143366 and the Japanese Laid-open Utility Model Publication No. 55-113570 are well known in the art. In the light emitting diode of the type shown in FIG. 7, the light emitting element 71 is positioned in alignment with the focal point of the reflecting surface 72 vapor deposited with aluminum or silver in a concave surface portion of the casing 71 or provided with a plated layer. This is similar to the first example described above as far as one end of the light emitting element 73 is fitted to a lead 74a by the use of an electroconductive bonding agent while the opposite end is electrically connected with a lead 74b through a gold wire 75.
In those light emitting diodes, aluminum or silver is vapor deposited, or the plated layer 72 is provided, on the concave surface portion of the casing 71. Subsequently, the leads 74a and 74b are inserted into the casing 71 and, thereafter, the light emitting element 73 is connected at one end to the lead 74a by the use of an electroconductive bonding agent and at the other end to the lead 74b electrically through the gold wire 75.
Thereafter, after a transparent epoxy resin 76 is potted to fill up a concave portion of the reflecting surface 72, a heating is carried out to harden it to thereby fix the light emitting element 73. In this type of the light emitting diode, by filling the concave portion with the transparent epoxy resin 76, the relation in optical positions between the reflecting surface 72 and the light emitting element 73 becomes highly accurate and the light emitting diode having an excellent optical characteristic can be obtained and, since the molding process based on the potting method is employed, the light emitting diode can be marketed inexpensively.
As a third example in which the metallic reflecting layer prepared beforehand is employed, light emitting diodes disclosed in the Japanese Laid-open Patent Publication No. 55-118681 and US Published Patent Application No. 2001/0024087 are well known in the art. As shown in FIG. 8, in this type of the light emitting diode, aluminum or silver is vapor deposited or a plated layer is provided in a concave surface portion of a reflecting mirror 85 in the form of a metallic plate and a light emitting diode 81 is arranged so as to be positioned at the focal point of the reflecting mirror 85 in the form of the metallic plate. This is similar to the first example described above as far as one end of the light emitting element 81 is fitted to a lead 82a by the use of an electroconductive bonding agent while the opposite end is electrically connected with a lead 82b through a gold wire 83.
In those light emitting diode, the leads 82a and 82b, the light emitting element 81, the gold wire 83 and the reflecting mirror 85 in the form of the metallic plate vapor deposited with aluminum or silver or provided with the plated layer in the concave surface portion are integrated together by means of a transfer molding process using a transparent epoxy resin 84, followed by heating to harden. Even in this type of the light emitting diode, since the reflecting mirror 85 and the light emitting element 81 are encapsulated with the transparent epoxy resin 84, the relation in optical positions between the reflecting mirror 85 and the light emitting element 81 becomes highly accurate and the light emitting diode having an excellent optical characteristic can be obtained.
However, with the light emitting diode having the structure in the first example, in which the metallic reflecting surface is provided outside the casing, since the light emitting diodes are packaged for shipping with the reflecting surfaces exposed to the outside, even though the protective layer is employed in the form of a hard coating, contact between the reflecting surfaces and contact between the reflecting surface and tips of the leads will often bring about flaws appearing on the reflecting surface having been passed through the protective (hard coating) layer during surface mounting or handling of the light emitting diodes after being unpackaged. Because of the flaws, there have been problems in that the reflecting performance of the reflecting surface tends to be lowered and the reflecting surface tends to be deteriorated quickly.
It is to be noted that when electric or electronic component parts are to be surface mounted on a circuit board, the circuit board in its entirety is generally passed through the solder reflow furnace having an atmosphere of about 250° C. so that the component parts can be soldered to circuits on the circuit board. Accordingly, when the light emitting diode having the above described metallic reflecting surface is surface mounted on the circuit board and is passed through the solder reflow furnace, the circuit board including the light emitting diode is naturally heated to a temperature about equal to 250° C. As such, wrinkling and/or cracking may occur in the reflecting surface due to the difference in coefficient of thermal expansion between the transparent epoxy resin and the silver or aluminum metallic layer forming the reflecting surface, accompanied by reduction in reflectivity of the reflecting surface, which eventually leads to reduction in optical characteristics.
In addition, during transportation of the light emitting diodes, special package must be employed so that the reflecting surface of each of the light emitting diodes will not be impaired by the effect of vibrations.
In the case of the light emitting diode of the structure in which it is integrally molded only with the conventional epoxy resin, an additional problem has been found, in which an abnormal stress will act on end faces of the epoxy resin during bending of the leads protruding outwardly from the light emitting diode, which is effected at the time of surface mounting on the substrate, and cracking eventually occurs in portions of the epoxy resin where the leads extend.
On the other hand, In the second example in which the metallic reflecting surface is provided inside the casing, the transparent epoxy resin is filled within a concave portion of a concave reflecting body by means of the potting technique to thereby fix the light emitting element and the concave reflecting surface. However, when the transparent epoxy resin undergoes shrinkage as it hardens, it often occurs that wrinkling and cracking occurs in the concave reflecting surface due to the difference between the thermal expansion coefficient of the transparent epoxy resin and that of silver or aluminum used to form the concave reflecting surface. In the worst case, the concave reflecting surface may exfoliate with debris scattering into the epoxy resin. In such case, as is the case discussed hereinabove, a practically considerable problem arises in that the reflectivity decreases to such an extent as to lower the optical characteristic thereof. For this reason, the reflecting characteristic is in practice secured by employing a white ABS resin or the like having a high reflectivity with no metal used in the concave reflecting portion, the truth is that no sufficient reflectivity can be obtained.
In the third case in which the metallic reflecting layer prepared beforehand is employed, by integrating the light emitting element 81 and the reflecting mirror 85 in FIG. 8 together by the use of the transparent epoxy resin, the light emitting element and the concave reflecting surface are fixed together. However, in this third example, after the reflecting mirror 85 has been manufactured in a separate process step, the light emitting element 81 and the reflecting mirror 85 are integrated together by the use of the transparent epoxy resin and, therefore, the number of process steps increases, resulting in increase of the price of the light emitting diode.
Also, in the conventional light emitting diode in which the light emitting element and the reflecting mirror are integrated together by the use of the transparent epoxy resin, since the rate of shrinkage during the hardening is considerably great, as shown by reference numeral 68 in FIG. 6, reference numeral 78 in FIG. 7 and reference numeral 88 in FIG. 8, the light radiating surface after the hardening is not a flat surface, but a concave surface and, hence, brings about an effect similar to that afforded by the reflecting surface fitted with a concave lens and, accordingly, there has been a problem that rays of light are not reflected from the reflecting surface as specified by the optical design.